The present invention is directed to papermaking fabrics having shaped yarns, and more particularly to papermaking fabrics made with structural yarns shaped for enhanced lateral, vertical and torsional stability.
Papermaking fabrics are subjected to harsh environments through every stage of the papermaking process. Forming fabrics are exposed to high moisture and caustic chemicals in the forming section as the paper web is formed on the fabric surface. Press fabrics are also exposed to high moisture and high contact pressure in the nip between press rolls in the press section where the paper web is de-watered. Dryer fabrics are subject to both high heat and humidity in the dryer section as the paper web is dried to form the final paper product. All of these papermaking fabrics must maintain dimensional and structural stability throughout the fabric life in order to support the paper web. Movement through the various machine rolls causes a great potential for lateral (Y), vertical (Z) or torsional (rotation) displacement of circular yams typically used in papermaking fabrics.
Traditionally, papermaking fabrics were woven with yarns having circular cross-sections. Their uniform shape negated worries about rotational distortion-except for twist problems. These yarns, however, were subject to lateral displacement where one yarn slides toward another yarn, creating an open area and a bunched area, rather than uniform contact and open areas. The fabric therefore suffered from lack of control of contact surface area and permeability.
To combat lateral displacement of structural yarns, shaped yarns (such as I, U or X-shaped yarns) have been used to cradle structural yarns to lessen the amount of displacement relative to one another in multilayer fabrics. Yarns having X or Y shapes or otherwise having flexible leg portions have been used in the weft position to enhance permeability control and for stabilization against distortion. Shaped yarns have also been used as stuffer yams to fill voids in spiral fabrics to reduce the fabric permeability.
A common theme with the prior known shaped yarns is that they deform during the weaving process to lock themselves into position. In the deformed shape, these yarns are more stable and also act to stabilize the cross-direction yarns. This is seen in both hollow and shaped yarns. Many of these shaped yarns (X, I, H, Y) as well as hollow yams accomplish goals of increased stability, reduced displacement and permeability control. However, they do not directly address lateral, vertical and torsional displacement in the structural yarns of a papermaking fabric.
It would be desirable to provide a papermaking fabric which will withstand the ravages of a papermaking machine and resist lateral, vertical and torsional displacement.
Generally, the present invention is a papermaking fabric which has increased stability due to the use of beam construction yarns which resist lateral, vertical and torsional deflection as well as compression, when interwoven as structural yams. The yarns are generally non-circular in cross-section and are preferably I-beam, H-beam, box-beam, or a hybrid construction. Each type has certain advantages and disadvantages with the hybrid construction being capable of multiple combined advantages.